104 MAGNIFICATION AND MICROMETRY. [CH. IV. 



using this valuation for actual measurement, the tube of the microscope and the 

 objective must be exactly as when obtaining the valuation (see \ 165). 



Example of Measurement. — Suppose one uses the red blood corpuscles of a dog 

 or monkey, etc., every condition being as when the valuation was determined, one 

 notes very accurately how many of the graduations on the wheel are required to 

 make the movable line traverse the object from edge to edge. Suppose it requires 

 94 of the graduations to measure the diameter, the actual size of the corpuscle 

 would be 94 X .08/* = 7.52/i. 



The advantage of the filar micrometer is that the valuation of one graduation 

 being so small, even the smallest object to be measured would require several 

 graduations to measure it. In ocular micrometers with fixed lines, small ob- 

 jects like bacteria might not fill even one space, therefore estimations, not meas- 

 urements, must be made. For large objects, like most of the tissue elements, the 

 ocular micrometers with fixed lines answer very well, for the part which must be 

 estimated is relatively small, and the chance of error is correspondingly small. 



§ 164. Obtaining the Ocular Micrometer Valuation for an Oc- 

 ular Micrometer with Fixed Lines (Figs. 33, 34, p. 25). — Use the 

 stage micrometer as object. Light the field well and look into the mi- 

 croscope. The lines of the ocular micrometer should be very sharply 

 defined. If they are not raise or lower the eye-lens to make them so ; 

 that is, focus as with the simple magnifier. 



When the lines of the ocular micrometer are distinct, focus the mi- 

 croscope (§ 45, 46, 56) for the stage micrometer. The image of the 

 stage micrometer will appear to be directly under or upon the ocular 

 micrometer. 



Make the lines of the two micrometers parallel by rotating the ocular 

 or changing the position of the stage micrometer, or both if necessary, 

 and then make any two lines of the stage micrometer coincide with any 

 two on the ocular micrometer. To do this it may be necessary to pull 

 out the draw-tube a greater or less distance. See how many spaces 

 are included on each of the micrometers. 



Divide the value of the included space or spaces on the stage microm- 

 eter by the number of divisions on the ocular micrometer required to 

 include them, and the quotient so obtained will give the valuation of 

 the ocular micrometer in fractions of the unit of measure of the stage 

 micrometer. For example, suppose the millimeter is taken as the unit 

 for the stage micrometer and this unit is divided into spaces of y^th and 

 i^oth millimeter. If now, with a given optical combination and tube- 

 length, it requires 10 spaces on the ocular micrometer to include the 

 real image of y^th millimeter on the stage micrometer, obviously one 

 space on the ocular micrometer would include only one-tenth as much, 

 or yVth mm. -=- 10 = y^th mm. That is, each space on the ocular mi- 

 crometer would include T fo tn of a millimeter on the stage micrometer, 

 or T J o tn millimeter of length of any object under the microscope, the 

 conditions remaining the same. Or, in other words, it would require 



